Retroviruses are enveloped RNA viruses that, after infection of a host cell, reverse transcribe their RNA genomes into a DNA intermediate, or provirus. All viruses containing an RNA genome and producing an RNA-dependent DNA polymerase are contained in the retroviral family. The family is divided into three subfamilies: (1) Oncovirinae, including all the oncogenic retroviruses, and several closely related non-oncogenic viruses; (2) Lentivirinae, the "slow retroviruses" such as the human immunodeficiency virus (HIV) and visna virus; and (3) Spumavirinae, the "foamy" retroviruses that induce persistent infections, generally without causing any clinical disease. Retroviruses contain at least three types of proteins encoded by the viral genome, i.e., gag proteins (the group antigen internal structural proteins), pol proteins (the RNA-dependent DNA polymerase and the protease and integrase proteins), and env proteins (the viral envelope protein or proteins). In addition to genes encoding the gag, pol, and env proteins, the genome to the retrovirus includes two long terminal repeat (LTR) sequences, one at the 5' and one at the 3' end of the virus. These 5' and 3' LTRs promote transcription and polyadenylation of viral mRNAs and participate in the integration of the viral genome into the cellular DNA of the host.
The provirus can be stably integrated into the host's cellular DNA. Gene products encoded by the provirus are then expressed by the host cell to produce retroviral virions, thereby replicating the virus. Because the retroviral genome can be manipulated to include exogenous nucleotide sequence(s) of interest for expression in a target cell, retroviral vectors are important tools for stable gene transfer into mammalian cells.
Many proposed gene therapy applications use retroviral vectors to take advantage of the ability of these naturally infectious agents to transfer and efficiently express recombinant nucleotide sequences in susceptible target cells. Retroviral vectors suitable for use in such applications are generally defective retroviral vectors that are capable of infecting the target cell, reverse transcribing their RNA genomes, and integrating the reverse transcribed DNA into the target cell genome, but are incapable of replicating within the target cell to produce infectious retroviral particles (e.g., the retroviral genome transferred into the target cell is defective in gag, and/or in pol, see Coffin, J., In: RNA Tumor Viruses, Weiss, R. et al. (ed) Cold Spring Harbor Laboratory, Vol. 2, pp. 36-73, 1985).
Retroviral vectors and packaging cells (helper cells) have been developed to introduce recombinant nucleic acid molecules into mammalian cells without the danger of the production of replicating infectious virus. This methodology uses two components, a retroviral vector and a packaging cell. The retroviral vector contains long terminal repeats (LTRs), the foreign DNA to be transferred, and a packaging sequence. This retroviral vector will not reproduce by itself because the genes which encode the structural and envelope proteins are not included within the vector. The packaging cell contains genes encoding the gag, pol, and env proteins, but does not contain the packaging signal, so that the cell can only form empty virus particles by itself. With this method, the retroviral vector is introduced into the packaging cell, to create a cell able to produce virus. The cell manufactures viral particles containing only the retroviral vector DNA, and therefore has been considered safe.
Use of retroviral vectors is limited in many aspects. For example, although retroviruses can efficiently infect and stably integrate into the genome of rapidly-dividing cells, retroviral integration into the genome of non-dividing or slowly dividing cells is inefficient (Springett et al. 1989 J. Virol. 63:3865-3869; Miller et al. 1990 Mol. Cell. Biol. 10:4239-4242; Roe et al. 1993 EMBO J. 12:2099-2108). Most packaging systems provide only modest vector titers, and the fragility of retroviral vector particles complicate purification and concentration (Paul et al. 1993 Hum. Gene Therap. 4:609-615). Finally, retroviruses enter target cells by binding of retroviral envelope glycoproteins (encoded by the env gene) to specific target cell surface receptors. This envelope protein-cell surface receptor interaction is often species specific, and in some cases even tissue specific. Moreover, the level of expression of the cell surface receptor on the target cells can vary widely among target cells. As a result, retroviruses usually have a limited host range (Kavanaugh et al. 1994 Proc. Natl. Acad. Sci. USA 91:7071-7075; Hopkins 1993 Proc. Natl. Acad Sci. USA 90:8759-8760).
One strategy for both expanding retroviral host cell range and increasing the structural stability of the retroviral virion involves production of pseudotyped retroviral viral vectors. Pseudotyped retroviral vectors useful in transformation of target cells are generally composed of retroviral virion structural proteins (e.g, gag proteins), a recombinant RNA genome containing the nucleotide sequence of interest, the pol protein for reverse transcription of the recombinant RNA contained in the virion, and a non-retroviral envelope protein or an envelope protein from a different retrovirus. The recombinant RNA genome is usually replication defective, e.g., defective in the pol and/or gag genes, to prevent production of infectious retrovirus following transfer of the nucleotide sequence of interest into the target cell. The envelope protein of the pseudotyped retrovirus is normally selected to provide a broader host range or to provide selective targeting of cells to be infected.